Add a demo that interpolates a solution field

[jwallwork23]

To follow #26.

[jwallwork23]

Note related PR #53 added a demo which solves the PDE within the monitor function.

[jwallwork23]

Blocked by #114.

Edit: We can do this without utility monitors.

[jwallwork23]

Might be best to just add this as an extension at the end of https://mesh-adaptation.github.io/docs/demos/monge_ampere_helmholtz.py.html?

[jwallwork23]

Might be best to just add this as an extension at the end of https://mesh-adaptation.github.io/docs/demos/monge_ampere_helmholtz.py.html?

If so, we should add a mesh plot for the second case that's already in the demo because the plot is missing.